Fear makes dunderhead study (5)
I believe the answer is:
dread
'fear' is the definition.
('dread' can be a synonym of 'fear')
'dunderhead study' is the wordplay.
'dunderhead' becomes 'd' (head letter of 'dunder').
'study' becomes 'read' ('read' can be a synonym of 'study').
'd'+'read'='DREAD'
'makes' acts as a link.
(Other definitions for dread that I've seen before include "Terror" , "Show horror" , "Adder (anag)" , "quail" , "Disquiet" .)